The basic alchemical rule for In Nomine is that an alchemist spends a predetermined amount of time and materials making an alchemical recipe which contains the effect of a chosen ability. At the end of this time, he rolls against the necessary Alchemy skill number, and makes any rolls required to use the ability that is to have its effect embedded in the recipe. Assuming both sets of rolls pass, the alchemical object is successfully created. It loses its power either after use or after a short amount of time. How this is translated to GURPS will affect how alchemists appear in the game.
Alchemy(M/H) is the study of the interaction between material and celestial entities, with particular emphasis on observable changes in the material at the chemical level. Chemistry is not a prerequisite, but so much chemistry is learned in the process that fundamental operations of mundane Chemistry/TL, such as identifying materials, default in GURPS to Chemistry-3. In the modern day, this can make it point-effective for an alchemist to have a professional knowledge of Chemistry as well. In standard In Nomine, a GM may wish to allow an Alchemy roll, penalized at -1 or more, to perform mundane chemistry operations.
Alchemy is useless for creating formulae without the Enchantment skill; GMs may choose to require Enchantment as a prerequisite for Alchemy (alchemical texts are incomprehensible to someone without Enchantment) or allow the two to be bought separately (alchemical texts are apparently insane, but the mechanics of the operations are learnable and teachable). GURPS: In Nomine says (p. 175) that Alchemy requires Power Investiture (Sorcery), but this may be an error, since non-Sorcerors explicitly can use Alchemy in original In Nomine.
Enchantment(M/H) is the study of manipulations of Forces in their loose state. The skill is impossible to learn without Symphonic Awareness.
Curiously, while in original In Nomine the Enchantment skill cannot be learned with less than 6 Forces, the Corporeal Player's Guide mentions that some techniques can be used without Symphonic Awareness. Alchemy is already quite limited by the time required; if a GM wishes to press the vaguest limits of canon, allowing Enchantment skill to be learned without Symphonic Awareness solely for the purposes of alchemical recipes for skills or other mundane resources would make alchemy available to human players without the Soldier(or higher) package. Such alchemists would be performing mechanical rituals without truly understanding what they were doing, always dangerous.
Original In Nomine rules required 1 day of work per level, to a maximum of the alchemists' own level with the skill or Song, to create a recipe. There are various possibilities for time to be taken in GURPS: 1 day per actual skill level, which is very costly at low levels but after several weeks becomes much more efficient; 1 day per character point invested in a resource, which is efficient for inexpensive resources; and using the points-to-IN-level conversion chart on page 206 of GURPS: In Nomine, suitable for up to 8 to 16 points invested in skills and Songs. In standard In Nomine, Attunements always take 1 week; this rule may be imported, or the "1 day per character point" rule may be used, based on the attunement's cost in GURPS points. This would generally cause creation of such an item to take much longer than one week.
The base skill or Song stored in the recipe cannot be increased by taing extra time, though the final roll may be boosted by Essence or time spent if applicable. "Time for skill" may, however, be useful to increase Enchantment skill if the GM allows.
Besides the time and labor spent in creating alchemical formulas, materials are required. GURPS: Magic suggests rules for these which GMs may decide are applicable to their game.
The alchemist requires a lab. A home lab in the modern day would contain about $1000 worth of equipment and materials. The GURPS: Magic rules for Alchemy suggest that this is the minimal level of equipment necessary to perform alchemy reliably; this would allow an alchemist to keep their tools reasonably secluded in a back room of their house, but would asses a penalty to Alchemy skill (GM's discretion) for attempting alchemy with nothing but some snatched solutions in beakers over a burner.
More elaborate labs may require bonuses, or the GM may demand a higher level of minimal equipment. In the default rules for the modern day, a professional lab would contain about $5000 and give a +1 to Alchemy skill, and a high-tech lab would contain as much as $20,000 worth of equipment and grant a +2 to Alchemy skill.
Costs for alchemical items could vary widely, and are largely left for the GM as a means to exercise control of recipes. A cost of $50/day of work is in line with most formulas in GURPS: Magic. Higher-cost materials may grant bonuses to Alchemy skill; +1 for every doubling of materials cost might be suggested.
The rule for final creation of a recipe in standard In Nomine is to roll against Alchemy+Enchantment, rather unworkable in GURPS. Some conversion possibilities include rolling against Alchemy alone, with Enchantment as a prerequisite, or rolling against the lower of Alchemy and Enchantment, regardless of prerequisite status. Regardless of which mechanic is used, causing alchemical items to last for weeks equal to the degree of success on the creation roll before losing their charge will generally mesh well with standard In Nomine, weighted longer in GURPS.
It may be possible for various methods to preserve alchemical items, or restore their charge. These methods are fairly canonical, but might be abusive; however, each costs points and has their own limitations. As always, the GM is the final arbiter of whether any of these methods work. If they do, alchemists will find it much easier to make items for sale, and PCs may have a reasonable stock when beginning an adventure or an ally who regularly provides the PC with alchemical items "on retainer."
In Nomine canon generally holds that items secreted outside normal space do not experience decay due to the passage of time. If an alchemist Soldier or Saint is known to Laurence, his Scabbard Attunement may be automatically available to them. With this rule, an alchemical item secreted in Scabbard would lose no charge while stored. Scabbard can accommodate any number of items of the size of an alchemical recipe, but they must be weapons. What recipes qualify should be decided beforehand. Furthermore, a cost of 15 points is strongly recommended for the Attunement based on equivalent Advantages and Knacks, and the alchemist will likely be subjected occasionally to orders from angels of the Sword.
A relic can also be put into potentiality. However, making a mundane item into a relic requires the Celestial Song of Artifacts, and 50 Essence(thus 50 days of work) for the Feature "Summonable into the user's hand," which allows banishment into potentiality, so the original recipe had to last for at least 8 weeks. Such a relic recipe would cost 2.5 character points in addition to any costs demanded by the maker and preserver. Such recipes would be stored and used only for emergencies, depending upon the labor and Essence available. This is the harshest possible interpretation of the rule; alchemical "relics" essentially have the ultimate use limitation "One Use Only," which could reduce the necessary Essence, and thus time and character points, significantly.
Alternatively, a simple glass jar, relicized and made Summonable, could contain a mundane alchemical recipe and be banished into potentiality, to be resummoned and the recipe decanted when necessary. There is canon precedent for small incidental items to follow a holder into potentiality, such as clothing on a vanished Vessel. The jar (or multiple-item pouch) could then be re-used after each recipe was removed. (Thanks, Beth, for this suggestion.)
The Corporeal Song of Entropy can "youthen" people and objects; for alchemists this has the advantage of being learnable on their own and requiring fewer character points than other methods to preserve items if the Song can restore an alchemical item's charge back to its fresh state. On the other hand, this requires the alchemists' Essence on a regular basis, causes Disturbance, and risks unfavorable Interventions.
A specialized relic with a variant of the Corporeal Song of Entropy or the Song of Body Bag could hold an alchemical item indefinitely if the GM allows such things. An artificer capable of making relics is still required from which to obtain the relic bags, and capacity is limited by the size and number of such "bags" obtained (both at GM's discretion), but the method is sure and quiet.
In the pages that follow, standard Alchemy will be assumed for standard In Nomine, and the following summary of the basic Alchemy rules will be used for GURPS: